A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
In lithographic processes, it is frequently desirable to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment between two layers in a device. Various forms of inspection apparatuses (such as scatterometers) have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a diffraction “spectrum” from which a property of interest of the target can be determined.
Examples of known inspection apparatuses include angle-resolved scatterometers of the type described in United States patent application publication nos. US 2006-033921 and US 2010-201963, which documents are hereby incorporated by reference in their entirety. The targets used by such scatterometers are relatively large, e.g., 40 μm by 40 μm, gratings and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). Angle-resolved scatterometry can be combined with dark field imaging metrology, for example as described in U.S. patent application publication nos. US 2010-0328655 and US 2011-069292, which documents are hereby incorporated by reference in their entirety. Further developments of measurement techniques have been described in U.S. patent application publication nos. US 2011-0027704, US 2011-0043791, US 2011-102753, US 2012-0044470, US 2012-0123581, US 2013-0258310, and US 2013-0271740 and in PCT patent application publication no. WO 2013-178422, which documents are hereby incorporated by reference in their entirety. Dark field imaging enables the use of targets smaller than the illumination spot and targets which may be surrounded by product structures on a substrate. Multiple gratings can be measured in one image, using a composite grating target.
A significant parameter of a lithographic process which should be monitored is focus. There is a desire to integrate an ever-increasing number of electronic components in a device such as an integrated circuit. To realize this, the size of the components should be decreased and therefore the resolution of the projection system should be increased, so that increasingly smaller details, or line widths, can be projected on a target portion of the substrate. As the critical dimension (CD) in lithography shrinks, consistency of focus, both across a substrate and between substrates, becomes more significant. CD is the dimension of a feature or features (such as the gate width of a transistor) for which variations will cause undesirable variation in physical properties of the feature.